The present invention relates to a method and apparatus for inspecting the electric characteristics of a large number of semiconductor integrated circuits (hereinafter referred to simply as devices) that are formed on a semiconductor wafer (hereinafter referred to as a wafer).
As an example of this type of inspecting apparatus, reference will be made to an apparatus that comprises: a cassette mount section on which wafers are mounted in units of one cassette; a fork or forks for conveying the wafers from the cassette mount section to a loader section; a loader section from which the wafers are conveyed to a prober section; a prober section for measuring the electric characteristics of the devices of the wafers conveyed from the loader section; and a test section for inspecting the devices on the basis of the measurement results obtained in the prober section. In the loader section, a subchuck for pre-alignment is arranged, which aligns the wafers with one another by utilization of the orientation flats of the wafers when the wafers are conveyed by the fork. In the prober section, a main chuck, an alignment mechanism and a probe card are arranged. The main chuck is movable in the X-, Y-, Z- and .theta.-directions, and the wafers are mounted thereon, for measurement. The alignment mechanism positions the wafers on the susceptor with high accuracy. The probe card has contact elements (e.g., probes) which are brought into electric contact with the electrode pads of the devices of the wafers that have been positioned, and which output measurement results to the test section.
A test head provided with a tester is arranged in the test section. The tester is electrically connected to the contact elements of the probe card fitted on the head plate of the prober section, and electric signals are exchanged between the tester and the contact elements of the probe card, so as to measure the electric characteristics of the devices.
In the case where the electric characteristics of the devices that are arranged on a wafer in a matrix pattern are sequentially measured, the main chuck is index-fed according to a predetermined program. By this index feed, the devices arranged in the first row of the matrix pattern are sequentially measured. After the devices in the first row are measured, the devices arranged in the second row are measured in the direction opposite to that of the devices of the first row. This sequential measurement is repeated until the devices arranged in the last row are measured.
In the sequential measurement mentioned above, the devices on a wafer are inspected one by one. Needless to say, however, the devices may be inspected in groups of a predetermined number.
With reference to FIG. 6, a description will be given of a case where a heat-generating device D1, such as a logic element or another type of active element, is measured by the conventional inspecting apparatus. In this case, the device D1 generates heat when it is being measured. The heat from the device D1 is transmitted to the neighboring devices, with the result that the temperatures of the neighboring devices increase and become higher than the upper limit of the measurement start temperature range in which the measurement can be started. For this reason, the neighboring devices cannot be measured immediately after the measurement of the device D1. In other words, the neighboring devices must be measured after the temperatures thereof decrease to a temperature within the measurement start temperature range. This phenomenon will be described in more detail with reference to FIGS. 7A and 7B. After the measurement of device D shown in FIG. 7A is started, its temperature increases gradually, as shown in FIG. 7B. Simultaneous with this, the temperatures of devices 1 and 2 in the neighborhood of device D also increase. When the measurement of device D ends, the temperature of device 1 is higher than the upper limit of the measurement start temperature range. Therefore, measurement of device 1 cannot be started immediately after the end of the measurement of device 1. A certain time T is required before the wafer W is cooled by a cooling apparatus and the temperature of device 1 falls to a temperature within the measurement start temperature range. This time T is a loss time when measurement of device 1 cannot be measured. If such a loss time T is required after the measurement of each device, the total loss time will be very long before the end of the measurement of all devices, and the inspection efficiency is degraded.